Ocean Bottom Cable (OBC)
Seismic
At a glance
OBC
A method whereby, in contrast to
conventional marine seismic (towed
streamer) acquisition, acoustic
reflections are recorded by receivers
placed on the seabed.
Author : JIM, Mundy Obilor 13th Aug., 2011
Copyright © - All Rights Reserved.
Why OBC?
In addition to P-waves, mode-converted shear waves
(PS-waves) which do not travel in water are also
recorded. These mode converted records, which are
less-attenuated by gas than P-waves have proved to
be very useful in seismic imaging.
Basic Concepts:
1.
2.
3.
4.
Receivers are placed on the seafloor
Source vessel sails across the
cables/nodes
Possibility of using more than one
cable-laying vesel
Multi-component data (P- and Swaves, commonly 4C P,X,Y,Z) are
recorded and retrieved for QC and
processing. Sensors may be
permanently deployed for monitoring
purpose (OBN)!
P-waves are more attenuated in
gas than S-waves; S-wave passes
through An advantage!
Fig Source:The Leading Edge, 1999
The gains, the success stories
The gains from OBC surveys have been very remarkable
1.
2.
3.
Eliminates surface-related noise
(towing, streamer noise, etc)
Data quality improved by using
hydrophones and
geophones/accelerometers
Resolves several imaging challenges
confronted by streamer techniques
(gas clouds, carbonates, complex
structures, …)
Gas Cloud
An example from Tommeliten Field in
the Norwegian sector of the North Sea
(courtsey of Statoil). Here PS-wave
data is compared with PP data for
imaging through a gas cloud.
The Leading Edge Nov 1999
The gains, the success stories
1. Practical in areas where towed
streamer operations are difficult
(platforms, obstructed areas, etc)
2. Feasible in shallow waters
3. Improved resolution (broader
bandwidth)
P-wave
An example from Alba field: One
of the conclusions is that PSwave shows high-amplitude top
and base sand reflectors, not
observed on the P-wave data.
S-wave (improved imaging)
Fig.The Leading Edge Nov 1999
The challenges are fading…
Seabed Seismic has been faced with challenges which are continually
been overcome, among which are:
Cost: OBC surveys cost much more than streamer surveys but recently, improvements
in acquisition techniques have considerably reduced the average acquisition cost
Processing: software packages and tools adapted to shear wave data are on the
increase (bearing in mind the assymptotic nature of shear wave reflections)
Long cable requirements: Using more than one cable-laying vessel practically
reduces acquisition time increases acquisition rates reduces cost
Enormous Data size: S-waves travel at about half the speed of P-waves, so a
longer record length (sometimes10-12 sec) is required to record the converted
wave data depending on target deep. Hence you have large data size, and data
management capabilities. These have been overcome with inproved hardware
and fast computing technolgies good time management.
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The future is bright!
 Oil and gas companies--- both large and small, are increasingly
recognizing the values derived from using multi-component seismic
techniques. Indeed, the future is bright.
 Investment appears to be on the increase.
Teaser check it out!
VP 2TS
=
-1
VS TP
P- and S- wave reflections
Consider two hrizons A and B, and using the standard PP and PS time-ratio method,
1. Derive the equation shown above which relates the the Vp/Vs ratio as a function
of 2-way travel time (Hint: assume a distance d between the two horizons and
take Θ and Ø as interface reflected angles for P and S reflections)
1. Calculate the average interval Vp/Vs ratio if time-thickness between the two
horizons for PP and and PS data are 2000ms and 3800ms respectively
Author : JIM, Mundy Obilor 13th Aug., 2011
All Rights Reserved.
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